The field of the invention generally relates to automatic washing machines, and more particularly relates to washer suspensions that have multiple modes of motion.
As is well known, a typical automatic washing machine sequences through a spin cycle after completion of a wash or rinse cycle. During a spin cycle, the agitator and the perforated clothes basket are accelerated up to high speed rotation to extract moisture from the clothes by centrifugal force. When the clothes are unevenly or nonuniformly distributed around the walls of the clothes basket, out-of-balance forces are created. When these forces are transmitted to the base, or more particular the feet of the washing machine, undesirable vibration may occur and, in extreme cases, the washing machine may actually "walk".
A primary objective of a washing machine suspension is to minimize the out-of-balance or unbalance forces that are transmitted to the base. In particular, it is important to minimize the vertical forces transmitted to the feet because these are the dominant forces responsible for walking. Generally, a washer suspension system absorbs these out-of-balance forces by permitting the tub assembly to move resiliently within the cabinet. A somewhat conflicting objective of a suspension system is to limit the excursion of the tub assembly because an unduly large free motion envelope around the tub assembly necessitates an unduly large cabinet size. It is well understood that even though the maximum rotational speed may typically be 600-700 rpm, the maximum excursion generally occurs at a much lower rotational speed called the critical speed or frequency. For example, the critical speed may typically occur on the order of 100 rpm when the out-of-balance forces tend to be regenerative. Once through the critical speed, the excursion, or lateral distance of tub assembly motion, decreases.
In the most common type of upright automatic washing machine, the tub assembly is supported on a suspension system that has a fixed pivot point relatively close to the floor, and springs with relatively high rate (force needed to deflect per unit of distance) are used to bias the tub assembly towards its vertical or upright axis. When large out-of-balance forces exist, the tub assembly tilts from the fixed pivot point and moves in a circular motion. Such a suspension generally requires a relatively large cabinet to prevent mechanical interference that could cause collision damage to the cabinet or other components; alternatively, very high rate springs can be used, but such arrangement tends to lead to extreme vibration and walking.
Another type of washing machine suspension is described in U.S. patent application Ser. No. 633,816 filed Dec. 26, 1990. In the apparatus described therein, the tub assembly is supported at a mid-level of the washing machine, and the suspension permits motion of the tub assembly in two independent modes or characteristics. More specifically, an annular traversing member is supported on a mid-level support surface, and is free to traverse in sliding engagement in the horizontal or x,y plane. Further, the tub assembly has a downward extending dome that sits on a complimentary surface of the annular traversing member such that the tub assembly can pivot or tilt. Thus, the tub assembly can independently traverse or pivot. One set of springs is arranged to bias the tub assembly towards the center, and another set of springs urges the tub assembly towards its upright orientation. With such arrangement, the pivoting forces are reduced by the addition of the traversing motion, and lower pivoting forces result in lower vertical forces being transmitted to the feet. Thus, the washing machine has minimal tendency to walk. The pivoting forces are further reduced by providing a tub assembly that has a dynamic center of mass above the pivot point, and preferably at the approximate level of the out-of-balance weight which typically is 2 to 4 inches above the bottom of the clothes basket. In fact, when the dynamic center of mass coincides with the vertical level of the out-of-balance weight, the pivoting forces are theoretically zero. Thus, the only vertical forces on the feet would be moment forces resulting from the traversing motion. However, because the dynamic center of mass and the out-of-balance weight are both above the pivot point, the traversing and pivoting excursions generally occur in the same instantaneous radial direction, and therefore are additive. Thus, the cabinet has to be made large enough to accommodate a motion envelope for the additive excursions (i.e. traversing and pivoting).